The present invention concerns optical glasses particularly useful for producing ophthalmic lenses of high power.
The use of glasses with a high index of refraction (n.sub.d =1.7) in place of conventional glasses (n.sub.d =1.523) permits an increase in the radius of curvature and, as a consequence, a reduction in the thickness of the lens. This leads to two advantages: on the one hand, the weight of the lens is much lower (however the density of the lens must not be too high, typically less than 3.25 g/cm.sup.3 ; the conventional "flint" glass of high index containing BaO and PbO cannot be used, because of this reason) which provides better comfort for the wearer of eyeglasses, and, on the other hand, the aesthetic appearance is improved considerable. Generally, however, an increase in index leads to a decrease in the Abbe number (.nu..sub.d); that is to say, an increase in the dispersion of the glass. If the dispersion of the glass is too high, the chromatic aberration ("iridescence" at the edges of the lens, for example) becomes significant. As a consequence, one seeks to obtain, at the same time, a low density, a high index, and a high Abbe number (generally greater than 34).
The object of this invention is to produce glasses satisfying the above-stated criteria. In particular, they exhibit an index of refraction (n.sub.d) between about 1.699-1.703, an Abbe number equal to or greater than 41.0, and a density less than approximately 3.25 g/cm.sup.3. Their chemical durability in an acid medium, as demonstrated in the test described later, is excellent (the loss in weight is less than about 2000 mg/dm.sup.2). The transmission in the visible portion of the radiation spectrum is high (the transmission is equal to or greater than 75% at a wavelength of 400 nm and at a thickness of 10 nm), and their fabrication can be done in a continuous tank because of their good devitrification characteristics, i.e., their good resistance to devitrification.
Glasses satisfying the above-mentioned criteria of index, dispersion, and density have previously been described in U.S. Pat. No. 4,404,290 (French Pat. No. 82 12447, European Patent Application No. 99,736). More specifically, that patent concerns glasses having a typical example of the following chemical composition, expressed in weight percent on the oxide basis:
______________________________________ SiO.sub.2 19.5 CaO 21.05 Nb.sub.2 O.sub.5 5.9 B.sub.2 O.sub.3 20.9 La.sub.2 O.sub.3 12.6 TiO.sub.2 7.65 Al.sub.2 O.sub.3 8.9 ZrO.sub.2 3.0 As.sub.2 O.sub.3 0.5 ______________________________________
But under certain conditions it could be deemed that the chemical acid durability of the glass (exhibiting a weight loss greater than 10,000 mg/dm.sup.2) demands improvement, all while maintaining a .nu..sub.d equal to or greater than 41.0 and a density less than 3.25 g/cm.sup.3.